This invention relates to wireless synchronous time systems and, more particularly, to a system and method of increasing battery life of a remote battery-operated timekeeping device.
No matter how accurately two independent timekeeping devices (e.g., clocks) are initially calibrated and synchronized, the devices will over time deviate from each other. In order for clocks to maintain accuracy within specified limits, the clocks should be periodically synchronized to an accurate reference time. The maximum period between these synchronizations is dependent on the required accuracy and rate of timekeeping variation allowed for an individual clock. In the case of remote battery-operated wireless timekeeping devices, such synchronizations consume extra current from a battery and, thereby, shorten battery life. The amount of battery life decrease per synchronization varies depending on the particulars of the timekeeping device and its synchronization system. In some devices and systems, each synchronization may shorten the battery life by 5 hours to 50 hours. Therefore, to maximize battery life in a wireless timekeeping device, the number of synchronizations should be kept to a minimum.
It is typically acceptable for the accuracy of a timekeeping device to vary for different uses, functions, or circumstances. For example, for a watch or a household clock, an accuracy of ±1 minute may be acceptable. However, for a time clock, a radio station, or a national standard, the same accuracy is usually not acceptable. Furthermore, the required accuracy of a timekeeping device may be permitted to vary depending on current circumstances or conditions without causing a negative impact. For example, the required accuracy of a school clock on school days when students are present is greater than the required accuracy of the school clock on non-school days (e.g., weekends, vacations, etc.). In addition, synchronized clocks within a particular facility may not require the same accuracy. For example, a timekeeping device for ringing a school bell and operating school clocks with second hands may require synchronization within ±¼ second so that the clocks visually synchronize with bell ringing. In contrast, clocks without a second hand may be permitted to vary by ±5 seconds because visual synchronization is not apparent.
Previous methods have been developed to help improve free running accuracy of a timekeeping device. U.S. Pat. No. 4,448,543 discloses improving accuracy by compensating for changes in temperature; U.S. Pat. No. 4,899,117 discloses improving accuracy by compensating for crystal aging and temperature; and U.S. Pat. No. 5,274,545 discloses improving accuracy by accounting for systematic and random variations. However, all of these developments are limited to the timekeeping units themselves and require compensation hardware and/or software to be located within the units. In addition, the timekeeping units are intended for fixed, rather than variable, timekeeping accuracy requirements. For wireless synchronized systems including hundreds of remote wireless timekeeping devices, one central master unit, and variable timekeeping requirements, these methods would involve duplication of hardware for each remote device. These methods would also limit free running timekeeping accuracy and synchronization control to the hardware and/or software of each individual remote device. Furthermore, none of these methods disclose or suggest a system or method to increase battery life by adjusting individual timekeeping requirements to variable requirements that change based on, for example, time of day, a particular device's environment, or operating conditions of a particular device.